This invention relates to an image forming method including the step of developing electrostatic latent images formed in electrophotography, electrostatic recording, electrostatic printing, etc. by use of an organic photoconductive photosensitive member with a two-component developer.
At present, for formation of a visible image from a certain image information, a method through electrostatic latent images such as electrophotography, etc. has been widely used. For example, according to an example of electrophotography, electrostatic latent images formed on a latent image bearing member comprising a photoconductive photosensitive member by the charging step and the exposure step is developed with a developer comprising electroscopic colored particles called toner, and the toner image is ordinarily transferred onto a transfer material and fixed to give a visible image.
The developers to be used in development of such electrostatic latent images may be classified into the so-called two-component developer comprising a mixture of toner and carrier and the so-called one-component developer comprising a magnetic toner containing magnetic material which is to be used solely without being mixed with carrier. In the system employing the two-component developer, toner is subjected to triboelectric charging by stirring mechanically toner with carrier, whereby it is possible to control to a considerable extent the polarity of charging and the amount of charging of the toner by choosing the characteristics of the carrier, the conditions of stirring, etc. In this respect, the two-component developer is superior to the one-component developer.
The developing method includes the magnetic brush method, the cascade method, etc., of which the magnetic brush has preferably been employed. The magnetic brush method is a method, in which spikes of developer erected in a shape of brush by magnetic force on a developer conveying support, namely, magnetic brush, is formed and the magnetic brush is brushed against the surface of a latent image bearing member, thereby attaching toner particles onto electrostatic latent images to effect development.
In the developing method utilizing such a magnetic brush method, etc., the intensity of the electrical field in the developing region is one of the important conditions which determine the developing characteristic. And, as the factors defining such an intensity of the electrical field, there are the surface potential when the photoconductive photosensitive member constituting the latent image bearing member is charged, the distance between the latent image bearing member and the developer conveying support in the developing region and the electrical resistance of the developer (particularly carrier in the two-component developer), and the intensity of the electrical field is determined depending on the relationships among these factors.
On the other hand, it is recently desired to use an organic photoconductive photosensitive member, which is appreciated in not only high durability at high temperature and stable photosensitive characteristic over a long term but also in high safety with an additional advantage of low cost. However, in such an organic photoconductive photosensitive member, due to generally small photosensitivity, the potential difference between the non-image portion irradiated by light in the exposure step and the image portion not irradiated by light. For this reason, in the image forming method employing an organic photoconductive photosensitive in general, the density of the visible image obtained is disadvantageously low. Also, an organic photoconductive photosensitive member has very delicate characteristics and therefore slight fluctuation in developing conditions may have a great influence on the visible image, whereby there is involved the problem such that it is difficult to obtain excellent visible images stably.